Process for the stabilization of dicalciumphosphate dihydrate



United States Patent 3,411,873 PROCESS FOR THE STABILIZATION 0F DICALCIUMPHOSPHATE DIHYDRATE Heinz Harnisch, Lovenich, near Cologne, Joseph Cremer, Hermulheim, near Cologne, and Friedrich Schulte, Hurth, near Cologne, Germany, assignors to Knapsack Aktiengesellschaft, Knapsack, near Cologne, Germany, a corporation of Germany No Drawing. Filed Nov. 30, 1966, Ser. No. 597,875 Claims priority, application Germany, Dec. 22, 1965, K 57,982 11 Claims. (Cl. 23-109) ABSTRACT OF THE DISCLOSURE The present invention relates to a process for stabilizing dicalcinm phosphate dihydrate by means of a magnesium phosphate.

Dicalcium phosphate dihydrate, particularly dicalcium phosphate dihydrate used for cosmetic or medicinal preparations, is known to be stabilizable. The stabilization agents used for that purpose include, for example, tetrasodium-, calcium-, magnesiumas Well as calcium disodium-pyrophosphates, alkali metal tripolyphosphate, alkali metal hexametaphosphate, diand trimagnesiurn phosphates, which are used alone or in combination. Dicalcium phosphate dihydrate stabilized by means of these substances has however sometimes been found to have an unsatisfactory stability, especially for use in fluoride-containing tooth paste.

A method for stabilizing dicalcinm phosphate dihydrate by admixing therewith an effective stabilizing amount of a magnesium-ammonium phosphate. The stabilizing magnesium-ammonium phosphate can be combined with an alkali metal pyrophosphate or an alkaline earth metal pyrophosphate.

It has now unexpectedly been found that the stabilization of dicalcinm phosphate dihydrate can be improved by using a magnesium-ammonium phosphate, for example magnesium-ammonium phosphate hexahydrate, as the stabilizer which is preferably used in combination with an alkali metal or alkaline earth metal pyrophosphate. The dicalcium phosphate dihydrate, especially fine-grained material, should advantageously be stabilized by incorporating therewith 0.4 to 0.9% by weight, preferably 0.8% by weight, alkali metal or alkaline earth metal pyrophosphate and a proportion of magnesium-ammonium phosphate sufiicient to result in the formation of dicalcinm phosphate dihydrate with a MgO-content between 0.2 and at most 0.4% by weight, preferably of 0.3% by weight. The stabilizers can readily be introduced into the reactor during the production of the dicalcinm phosphate but before isolation thereof, at a pH-value between 7.5 and 8.5. When coarsely crystalline product formed, for example, of particles of which 45 to 90% by weight have a size of 45,u, is to stabilized, it is advantageous to grind at least a portion of the stabilizer together with solid, dry dicalcinm phosphate. In this event, the dicalcium phosphate is mixed during its preparation with 0.1 to 0.4% by weight, preferably with 0.3% by weight alkali metal or alkali earth metal, pyrophosphate, referred to the dicalcinm phosphate dihydrate, and with a proportion of magnesium-ammonium phosphate sufficient to result in the formation of dicalcinm phosphate dihydrate with a content of MgO between 0.2 and at most 0.4% by weight, preferably of 0.3% by weight, and the isolated and dried product is mixed subsequently with 0.4% by weight alkali metal or alkali earth metal pyrophosphate to be ground then into particles of predetermined size. The magnesium-ammonium phosphate proportion can 3,411,873 Patented Nov. 19, 1968 "ice also be mixed first with solid, dry dicalcinm phosphate, and the mixture ground subsequently.

A comparison of the stabilizing activity of magnesiumammonium phosphate monohydrate, magnesium-ammonium phosphate hexahydrate and dimagnesium phosphate trihydrate used in combination with alkali metal or alkali earth metal pyrophosphate shows (cf. the table below) that magnesium-ammonium phosphate monohydrate produces the best results.

The data set forth in the table below were obtained in a short time test taking no more than about 5 minutes, which was carried out as follows:

5 grams dicalcinm phosphate dihydrate stabilized inside the reactor with 0.8% by weight pyrophosphate were made with 20 grams water into a slurry, and the magnesium compound listed in the table below was then added in a proportion sufficient to ensure a MgO-content between 0.1 and 1.0%, referred to the phosphate. The pH-value of the mixture was determined at room temperature, and the sample was then heated to 100 C. with continuous agitation. After 5 minutes, counted from the start of heating, the mixture was cooled down to room temperature and the pH-value was determined again; the pH-value should not be lower than 5.5 for a good stabilizer.

5 minutes test of some Mg-phosphate stabilizers:

Quantity Stabilizer calculated pH pH in percent (20 C.) ("20 C MgO 0. 0 7. 1 4. 7 lUgNHrPOr-HaO 0. 1 7. 4 5. 45

IVIgNHAPOJ-GHQO 0.1 7. 5 5.2 0. 2 7. 6 5. 7 0. 3 7. 7 6. 1 0. 4 7. 8 6. 2 0. 5 7. 8 6. 2

NlgNH-iPOtGHJO 1.0 7. 9 6. 3

BIgHPO4-3Hz0 0.2 7. 4 5. 6 0. 3 7. 4 6. 0 0. 4 7. 5 6. O 0. 5 7. 6 6. 2

1 After heating.

The stabilization achieved by means of magnesiumammonium phosphates has been found in the short time test to produce pH-values more favorable than those obtained with comparative quantities of conventional magnesium phosphate stabilizers.

A further advantage associated with the process of the present invention resides in the fact that considerably lesser proportions of alkali metal or alkaline earth metal pyrophosphate are needed for producing the stabilization.

EXAMPLE 1 A reactor provided with a rapid propeller stirrer was filled with 220 liters Water. 100 kg. CaCO were simultaneously suspended in a second container; the CaCO suspension had a density of about 1.3 to 1.5 g./cc. In a first step, the carbonate suspension and phosphoric acid having a strength of 70 to by weight were simultaneously pumped into the reactor. The phosphoric acid was supplied at a rate such that the pH-value varied between 2.3 and 2.6. As soon as all of the carbonate was found to have reacted, the pH-value was finally adjusted in a second step by reaction of the phosphoric acid in excess with calcium chloride (25-35% strength by weight) and sodium hydroxide solution (20S0% strength by weight) in stoichiometric proportions. The temperature inside the reactor was found to increase from initially 2530 C. (first step) to 40-45 C. At a pH-value of 8.0, the supply of calcium chloride and sodium hydroxide solution was arrested, and the material was stabilized subsequently by adding 0.3% MgO in the form of and Na4P207.

The dicalcium phosphate was filtered off and Washed. The filter cake was dried in a fluidized bed drier, mixed with 0.4% by weight sodium pyrophosphate and ground in a sifter bowl mill to obtain particles of which 99.5% had a size of 40,u. The sodium pyrophosphate should be used in the form of particles of which about 60 to 80% have a size of 40 i.e., a size corresponding to that of the dicalcium phosphate obtained.

EXAMPLE 2 The procedure and conditions were the same as those used in Example 1, save that the MgNH PO -H O was replaced with an equivalent proportion of M gNH PO 6H O EXAMPLE 3 The reaction procedure and conditions were the same as those used in Example 1, save that the 0.3% by weight MgNH PO -H O were mixed with dried dicalcium phosphate and ground, rather than introduced into the reactor at a pH-value of 8.0. The magnesium phosphate should be used in the form of particles of which about 45 to 90% have a size of 45p., i.e., a size corresponding to that of the dicalcium phosphate obtained.

EXAMPLE 4 The procedure and conditions were analogous to those used in Example 3, save that the MgNH PO -H O was replaced with an equivalent proportion of her selected from the group consisting of alkali metal pyrophosphate and alkaline earth metal pyrophosphate as a stabilizer.

5. The process of claim 1 wherein stabilization is effected by admixing 0.40.9% by weight of an alkali metal or alkaline earth metal pyrophosphate with an amount of magnesium-ammonium phosphate suflicient to effect formation of dicalcium phosphate dihydrate having a MgO content of 0.20.4% by weight.

6. The process of claim 5 wherein dicalcium phosphate dihydrate is stabilized by adding 0.8% by weight pyrophosphate and an amount of magnesium-ammonium phosphate sufiicient to effect formation of dicalcium phosphate dihydrate having a MgO content of 0.3% by weight.

7. The process of claim 1 wherein a stabilizer is introduced into the reaction zone during the preparation of the dicalcium phosphate at a pH value of 7.5-8.5.

8. The process of claim 1 wherein part of the stabilizer is ground together with solid dry dicalcium phosphate.

9. The process of claim 8 wherein 0.1-0.4% by weight pyrophosphate, referred to the dicalcium phosphate dihydrate, and magnesium-ammonium phosphate sufiicient to effect formation of dicalcium phosphate dihydrate with a MgO-content between 0.20.4% by weight, are introduced as the stabilizers into the reaction zone, the resulting isolated dried product being subsequently mixed with 0.4% by weight pyrophosphate and ground into particles of predetermined size.

10. The process of claim 9 wherein 0.3% by weight pyrophosphate, referred to the dicalcium phosphate dihydrate, and magnesium-ammonium phosphate sufiicient to result in the formation of dicalcium phosphate dihydrate with a MgO content of 0.3% by weight are introduced as the stabilizers into the reaction zonc, the isolated and dried product being subsequently mixed with 0.4% by weight pyrophosphate and then ground into particles of predetermined size.

11. The process of claim 8 wherein magnesium-ammonium phosphate is first mixed with solid dry dicalcium phosphate, the mixture being thereafter ground into particles of predetermined size.

No references cited.

EARL C. THOMAS, Primary Examiner.

L. A. MARSH, Assistant Examiner. 

